Version 1.1

Updated December 2007

NOTE:
This Open-File Report represents one preliminary part of a larger planned series of integrated geologic databases that will ultimately be available for the entire United States. This interim version is being released now in order to provide ready access to standardized geologic data for use in regional analyses and to meet product distribution goals. The final compilation of these state databases will allow integration of the data that are found on state-scale geologic maps, presented in a uniform database structure. This Open-File Report is similar to but will not be identical to the final version of these data.

Introduction

The growth in the use of Geographic Information Systems (GIS) has highlighted the need for regional and national digital geologic maps attributed with age and lithology information. Such maps can be conveniently used to generate derivative maps for purposes including mineral-resource assessment, metallogenic studies, tectonic studies, and environmental research. This Open-File Report is a preliminary version of part of a series of integrated state geologic map databases that cover the entire United States.

The only national-scale digital geologic maps that portray most or all of the United States for the conterminous U.S. are the digital version of the King and Beikman (1974a, b) map at a scale of 1:2,500,000, as digitized by Schruben and others (1994) and the generalized digital version (Reed and Bush, 2004) of the Geologic Map of North America (Reed and others, 2005a, b) compiled at a scale of 1:5,000,000. The present series of maps is intended to provide the next step in increased detail. State geologic maps that range in scale from 1:100,000 to 1:1,000,000 are available for most of the country, and digital versions of these state maps are the basis for this product. In a few cases, new digital compilations were prepared (e.g. Ohio, South Carolina, South Dakota) or existing paper maps were digitized (e.g. Kentucky, Texas). Also as part of this series, new regional maps for Alaska and Hawaii are being compiled and ultimately new state maps will be produced.

The digital geologic maps are presented in standardized formats as ARC/INFO export (.e00) files and as ArcView shape (.shp) files. Accompanying these spatial databases are a set of five supplemental attribute tables that relate the map units to detailed lithologic and age information. The maps for the CONUS have been fitted to a common set of state boundaries based on the 1:100,000 topographic map series of the United States Geological Survey (USGS). When the individual state maps are merged, the combined attribute tables can be used directly with the merged maps to make derivative maps. No attempt has been made to reconcile differences in mapped geology across state lines.

Procedures

The various digital geologic maps that form the basis for this product were originally produced in a wide variety of formats. Although most of them are available as ARC/INFO export (.e00) files and/or ArcView shape (.shp) files, the items and formats in the polygon (PATs) and arc (AATs) attribute tables vary dramatically. To unify these disparate maps, it was necessary to create a set of standard formats, and then to convert the state digital geologic maps to conform to these standards. The details of these standards are presented in the documentation of this report. The creation of a unique map unit name, called unit_link, allows the different State maps to be merged. (Database field names are in italics). Unit_link consists of a two-letter State (ST) code, concatenated with the original geologic map unit name, which may be slightly modified to remove special characters for age designations, followed by a semicolon, and an integer that designates geographic regions (or provinces) within the map. For those states where provincial structure is absent, this integer is simply "0". This variable, unit_link, can then be used as a key field to relate the tables that contain age and lithologic information to the spatial database (.dbf).

Compilation of a regional geologic map always requires compromises between the complexity of geologic information for a large region, and the need to keep the compiled map, and its explanation, relatively simple. Similarly, compromises are necessarily made in order to convert the large variety of formats in our source maps into the standard set of formats developed for this series.

Typically, spatial databases were modified from the source in the following general manner: The most recent data was obtained and the arcs and polygons were reattributed in the PAT according to the nomenclature adopted for this series (see documentation in this report and the metadata for individual spatial databases). When this reattribution was complete, the other attributes from the original spatial databases were deleted. Generally, if faults were not an integral part of the spatial database, arcs were retagged to make them so. Additionally, for those states where faults were mapped, fault arcs were extracted and provided as a separate .e00 or shape file (faults_dd, faults_lcc).

A second more detailed set of standardized attribute tables was generated by extracting information from the legends of the source maps and entering it into a set of five tables that record map unit information (STunits), lithologic information (STlith), age information (STage), and references (STref and STref-link). Some existing map legends provided an inadequate level of age or lithologic information. In these cases, we consulted the scientific literature, maps at smaller scales, and, in some cases, the original authors of the compilations or other regional experts. When we used updated information, it was recorded in the STage and STlith tables. Thus, the age and lithology information in these attribute tables may, in some cases, conflict with the information on the legends of the original source maps that may have been compiled decades ago.

In particular, the lithology table (STlith) may be much more extensive than the information in the map legends. Large regional compilations like these state maps often utilize map units that encompass a variety of lithologies. Volcanic rocks are commonly “lumped” extensively, combining tuffs, ash-flow tuffs, flows, and subvolcanic intrusions of a number of compositions, so that individual map units may contain dozens of unique lithologies. Although a dominant (most abundant) lithology has been designated for all map units, users are advised to be cautious, as many map units simply have no single dominant lithology.

Bedrock state geologic maps are typically compiled on the basis of lithostratigraphic units. In areas of variable metamorphic grade, this leads to inherent difficulties when trying to classify these units by rock type. For instance, a single stratigraphic unit may contain rock types ranging from shale to gneiss or limestone to marble. Commonly these gradations are not noted by internal contacts within a stratigraphic unit on the geologic map. However, the stratigraphic unit descriptions found in map explanations may include both metamorphic and non-metamorphic rock names. This combination results in apparent inconsistencies when assigning rock types within a single stratigraphic unit and may be particularly notable when a stratigraphic unit crosses a state boundary.

Our approach to assigning rock types to the lithostratigraphic units was to begin by using the information provided by the state in the map explanation. If information was lacking, other references including the USGS National Geologic Map Database “Geolex” website ( http://ngmdb.usgs.gov/Geolex/geolex_home.html ) were used. This website is an on-line search tool which provides usage, location, type locality, unit descriptions, age and bibliographic entries for lithologic and geochronologic unit names.

Error correction is an ongoing process with most spatial databases. A typical state spatial database consists of tens of thousands of polygons and arcs, and errors introduced during the creation of these spatial databases are inescapable. One common type of error is data coding that does not conform to the original paper map. For example, polygons may be given the wrong map unit, or faults may be called normal faults instead of thrust faults. Some of these errors are unavoidable because, in a few cases, the original paper map cannot be interpreted with certainty.

In other cases, newer information is available that can be used to better describe the existing polygons and arcs that are based on decades-old compilations. This type of updating was done where necessary for some of the spatial databases. Any changes to the spatial data are documented (STchanges.txt where ST stands for the two-letter abbreviation for a given state.)

Schruben, P.G., Arndt, R.E., and Bawiec, W.J., 1994, Geology of the Conterminous United States at 1:2,500,000 Scale — A Digital Representation of the 1974 P.B. King and H.M. Beikman Map, U.S. Geological Survey Digital Data Series 11, release 2. (URL http://pubs.usgs.gov/dds/dds11/)

To download files, right click on the file that you want and either Save target as...(Internet Explorer) or Save link as...(Netscape).

Connecticut

The paper geologic map of the State of Connecticut was published by the Connecticut Geological and Natural History Survey in cooperation with the United States Geological Survey in 1985 (John Rodgers, 1985) at a scale of 1:125,000. This map is available through the Department of Environmental Protection (DEP) Store at http://dep.state.ct.us/store/index.htm.

In 2000 the Connecticut Geological and Natural History Survey published a digital version of the 1985 paper map. This digital version was prepared from a set of 1:50,000 scale regional sheets that were originally used to prepare the 1:125,000 scale paper map. The digital data are available for download from http://dep.state.ct.us/gis/dataguides/dep/layers/bedrock.htm along with a complete description of the bedrock mapping units. Additional data layers including surficial materials and Quaternary geology are available from http://dep.state.ct.us/gis/Data/data.asp.

Data Modifications

The spatial data for Connecticut in this report have been modified in several important ways. The description and referencing of these changes is tabulated in the file metadata.

The original coverage boundary was replaced with the standardized common state boundary derived from Digital Line Graphics (DLG) files: thus, one or more polygons for coastal water were added to fill in the area between the physical coastline and an off-shore state boundary.

In order for the symbology for linear features that have polarity, such as thrust faults, to display correctly, the polarity of the spatial feature must be known and correct. A total of 794 arcs were “flipped” to ensure correct polarity. To ensure conformity with the original paper map, two arcs were added, one arc was retagged, and one additional arc was flipped and retagged.

Notes on rock type attribution

For a review of how rock types were coded, see above and an additional discussion in the documentation.

In Connecticut, the map explanation describes some stratigraphic units that contain both metamorphic and non-metamorphic rock types. For example, the map description of the Stockbridge Marble includes metamorphic rocks (marble, phyllite, and schist) along with sedimentary rocks (siltstone and sandstone,). We assigned the unit the rock type names “marble”, “phyllite”, “schist”, “siltstone”, and “sandstone”. Thus, the lithologic coding for these units will show apparent inconsistencies in metamorphic grade.

Maine

The paper bedrock geologic map of the State of Maine was published by the Maine Geological Survey in 1985 (Osberg and others, 1985) at a scale of 1:500,000. In 1987, the U.S. Geological Survey scanned the original 1:500,000 scale mylar compilation sheets in order to generate a digital coverage. In 1990 bedrock unit attributes were added, and in 1994 the Maine Geological Survey added major faults and their attributes. This digital dataset is available from Maine Geographic Information Systems clearinghouse at http://apollo.ogis.state.me.us/catalog/catalog.asp?state=2&extent=cover#bedrock. These data are reproduced here with the permission of Dr. Robert Marvinney, State Geologist of Maine.

The spatial data for Maine in this report have been modified in several important ways. The description and referencing of these changes is tabulated in the file metadata.

The original coverage boundary was replaced with the standardized common state boundary derived from Digital Line Graphics (DLG) files: thus, one or more polygons for coastal water were added to fill in the area between the physical coastline and an off-shore state boundary.

In order for the symbology for linear features that have polarity, such as thrust faults, to display correctly, the polarity of the spatial feature must be known and correct. A total of 384 arcs were “flipped” to ensure correct polarity. To ensure conformity with the original paper map, 22 arcs were retagged, and one additional arc was flipped and retagged.

Notes on rock type attribution

For a review of how rock types were coded, see above and an additional discussion in the documentation.

In Maine, the bedrock state geologic map was compiled showing the interpreted protolith of the lithostratigraphic unit rather than the present rock type. Variations in metamorphic grade are illustrated by an isograd map of the state rather than in descriptions of present rock types. The protolith descriptions were used to assign rock types to stratigraphic units. Thus, there are apparent inconsistencies among rock types within units, particularly with respect to metamorphic grade, at the state boundary.

Massachusetts

In 1983 the U.S. Geological Survey published the paper version of the bedrock geologic map of Massachusetts (Zen and others, 1983) at a scale of 1:250,000. The original paper map had been created by compiling data in two pieces (the eastern half of the state and the western half of the state). Each half had a different central meridian but the two pieces were seamlessly merged along their common boundary to form a complete state map. In the late 1990s, the USGS contracted with Dr. Rudi Hon of Boston College to create a digital version from a paper copy of the map (the original mylars had been lost some years earlier). During that process the projection discrepancy came to light. An attempt was made at Boston College to resolve the registration problems by identifying about 1200 individual points, comparing them to the topographic and geographic base, and then adjusting the geologic linework incrementally to create a better match between geology and geography. The corrected map was delivered to the USGS in 1998.

The spatial data for Massachusetts in this report have been modified in several important ways. The description and referencing of these changes is tabulated in the file metadata.

The original coverage boundary was replaced with the standardized common state boundary derived from Digital Line Graphics (DLG) files: thus, one or more polygons for coastal water were added to fill in the area between the physical coastline and an off-shore state boundary.

In order for the symbology for linear features that have polarity, such as thrust faults, to display correctly, the polarity of the spatial feature must be known and correct. A total of 1382 arcs were “flipped” to ensure correct polarity. To ensure conformity with the original paper map, one arc (a polygon) was added, 265 arcs were retagged, and 59 additional arcs were flipped and retagged. An additional 34 polygons were retagged to conform to the paper map.

Notes on rock type attribution

For a review of how rock types were coded, see above and an additional discussion in the documentation.

In Massachusetts, the map explanation describes some stratigraphic units that contain both metamorphic and non-metamorphic rock types. As an example, the map description for a unit within the Walloomsac formation lists phyllite, schist (metamorphic rocks) and limestone (sedimentary rock). We assigned the rock types “phyllite”, “schist”, and “limestone” to the unit. Thus, the lithologic coding for these units will show apparent inconsistencies in metamorphic grade, particularly with respect to carbonate units which tend to be described as their unmetamorphosed equivalent.

New Hampshire

The most recent paper state geologic map of New Hampshire was published in 1997 by (Lyons and others, 1997) at a scale of 1:250,000. This map was originally created from a digital file generated by the Complex Systems Research Center at the University of New Hampshire. In 2006 the digital file was updated by the New Hampshire Geological Survey and subsequently released with additional files in CD format as a part of the USGS Data Series (Bennett and others, 2006). The CD is available from the New Hampshire Geological Survey and the USGS.

The spatial data for New Hampshire in this report have been modified in several important ways. The description and referencing of these changes is tabulated in the file metadata.

The original coverage boundary was replaced with the standardized common state boundary derived from Digital Line Graphics (DLG) files: thus, one or more polygons for coastal water were added to fill in the area between the physical coastline and an off-shore state boundary.

In order for the symbology for linear features that have polarity, such as thrust faults, to display correctly, the polarity of the spatial feature must be known and correct. A total of 382 arcs were “flipped” to ensure correct polarity.

To ensure conformity with the original paper maps, six arcs were added and six arcs were retagged.

Notes on rock type attribution

For a review of how rock types were coded, see above and an additional discussion in the documentation.

In New Hampshire, rock types were assigned based on both the map explanation and additional information from the USGS GEOLEX website ( http://ngmdb.usgs.gov/Geolex/geolex_home.html ). A few of these descriptions contain both metamorphic and non-metamorphic rock types. For example, the map description for the Fitch formation includes “metamorphosed limestone, calcareous sandstone, siltstone, and dark pelitic schist”. We assigned the rock types “marble”, “quartzite”, “phyllite”, and “pelitic schist” to this unit. Thus, the lithologic coding for this unit will show apparent inconsistencies between the map description which used protolith rock type names and the rock types assigned which were the metamorphosed equivalents of those protoliths.

New Jersey

The most recent paper geologic map of New Jersey was produced at a scale of 1:100,000 by the New Jersey Geological Survey (NJGS) in cooperation with the U.S. Geological Survey (Drake and others, 1996; Owens and others, 1998). In addition, surficial geology for the state was compiled at 1:100,000 in another series of recent maps (Newell and others (2000) and Stone and others (2002)).

The 1:100,000 scale bedrock geologic map of New Jersey (Drake and others, 1996; Owens and others, 1998) was digitized by the New Jersey Geological Survey in 1999. These data are reproduced here with the permission of Karl Muessig, State Geologist of New Jersey. The spatial data are available as shape files (.shp) from the NJGS website at http://www.state.nj.us/dep/njgs/index.html and as ArcInfo export files (.e00) from the NJGS CD publication CD 00-1.

The spatial data for New Jersey in this report have been modified in several important ways. The description and referencing of these changes is tabulated in the file metadata.

The original coverage boundary was replaced with the standardized common state boundary derived from Digital Line Graphics (DLG) files: thus, one or more polygons for coastal water were added to fill in the area between the physical coastline and an off-shore state boundary.

In order for the symbology for linear features that have polarity, such as thrust faults, to display correctly, the polarity of the spatial feature must be known and correct. A total of 1781 arcs were “flipped” to ensure correct polarity. To ensure conformity with the original paper maps, one arc was added, 1728 arcs were retagged, and 98 additional arcs were flipped and retagged. In addition, 9 arcs (faults) were split to conform to the fault designations on the paper maps. One polygon was retagged to conform to the printed map and unit description.

In addition, a dike file was generated by heads-up digitizing three different dike sets (Jd, SObl, and Zd) from the paper map. These arcs had not been included in the original digital spatial file, but were identified and described on the paper map.

Note on rock type attribution

For a review of how rock types were coded, see above and an additional discussion in the documentation.

In New Jersey, rock types were assigned based on the detailed map explanation and A few of these descriptions contain both metamorphic and non-metamorphic rock types. For example, the map description for the Hardyston Quartzite includes quartzite (a metamorphic rock) as well as arkose, sandstone and conglomerate (sedimentary rock types). We assigned the rock types “quartzite”, “arkose”, “sandstone” and “conglomerate” to this unit. Thus, the lithologic coding for this unit will show apparent inconsistencies in metamorphic grade.

Rhode Island

The paper map of the bedrock of Rhode Island was published in 1971 by A. W. Quinn at a scale of 1:125,000. In 1994, the Rhode Island Geological Survey (Hermes and others, 1994) digitized the 1971 map, rescaled to 1:100,000, and incorporated additional data accumulated since the 1971 map was published. The bedrock spatial data can be freely downloaded from http://www.edc.uri.edu/rigis-spf/rigis.html. These data are reproduced here with the permission of Jon Boothroyd, State Geologist of Rhode Island.

Additional statewide digital coverages available from the same site include glacial geology, barrier beaches, hydrology, and soils. Information about other products about Rhode Island can be found at http://www.edc.uri.edu/rigis/.

Data Modifications

The spatial data for Rhode Island in this report have been modified in several important ways. The description and referencing of these changes is tabulated in the file metadata.

The original coverage boundary was replaced with the standardized common state boundary derived from Digital Line Graphics (DLG) files: thus, one or more polygons for coastal water were added to fill in the area between the physical coastline and an off-shore state boundary.

In order for the symbology for linear features that have polarity, such as thrust faults, to display correctly, the polarity of the spatial feature must be known and correct. A total of 11 arcs were “flipped” to ensure correct polarity.

Notes on rock type attribution

For a review of how rock types were coded, see above and an additional discussion in the documentation.

In Rhode Island, the map explanation describes some stratigraphic units that contain both metamorphic and non-metamorphic rock types. For example, the Conanicut Group – Fort Burnside formation includes phyllite (metamorphic rock) as well as siltstone (clastic rock). We assigned both rock types to the unit. Thus, the lithologic coding for this unit will show apparent inconsistencies in metamorphic grade.

Vermont

The paper geologic map of the State of Vermont was published by the Vermont Geological Survey in 1961 (Doll, 1961) at a scale of 1:250,000. This map is currently out of print; however, it is available from the Vermont Geological Survey as an image file on CD ROM or as a tiled image on line at http://www.anr.state.vt.us/dec/geo/centmap.htm.

In 2006 a georeferenced scanned version of a paper copy of the 1961 Vermont state geologic map was used to produce the digital version of the map. The digitizing was conducted at the USGS in cooperation with the Vermont Geological Survey. Additional review by the Vermont Geological Survey of the preliminary spatial files provided in this report is on-going.

The Vermont Geological Survey is undertaking a program of mapping at 1:24,000 and is currently working towards a new State bedrock map at a scale of 1:100,000 in cooperation with the USGS. For up-to-date information on the progress of this effort, check the State Bedrock Map Program page for the Vermont Geological Survey at http://www.anr.state.vt.us/dec/geo/statemap.htm.

For a review of how rock types were coded, see above and an additional discussion in the documentation.

In Vermont, the map explanation describes some stratigraphic units that contain both metamorphic and non-metamorphic rock types. As an example, the description for the Dalton formation includes schistose quartzite (metamorphic rock), impure dolomite and conglomerate (sedimentary rocks). We assigned the rock types “quartzite”, “dolostone (dolomite)” and “conglomerate” to the unit. Thus, the lithologic coding for these units will show apparent inconsistencies in metamorphic grade, particularly with respect to carbonate units which tend to be described as their unmetamorphosed equivalent.